Elastic and Inelastic Collisions in One Dimension - Need Help, due tonight

In summary: MaVa’Where:P = momentum (in Newtons)S = distance (meters)M = mass (kilograms)A collision is inelastic if the kinetic energy is not conserved.
  • #1
Princess19
36
0
I have a lab report due tonight. We were working on these in lab and I'm not sure how to tell which one was elastic or inelastic. We had three cases. We had to use the conservation of momentum and kinetic energy formulas.A ) Cart A and B have equal masses. Car B is at rest. Car A strikes car B. They don't stick together. Photogate measures different final velocities for each. Conservation of momentum -> 26% difference, and conservation of kinetic energy was 49 % difference.

B) Cart A has larger mass than Cart B. Cart B is at rest. Car A strikes Car B, and again they measure different velocities. Percent difference for momentum was 8.9 % and for kinetic energy it was 26 %.

C.1) Cart A and Cart B have equal masses, but Cart A is inverted, so that the two magnets on each car attract each other, ( I think we are simulating an elastic collision here, but I am not sure). Cart B is at rest. Cart A strikes Cart B and they stick together, both presenting with same final velocities. Momentum % diff. was 8.7% and kinetic energy was 57 %.

C.2)After Cart A, passes the photogate, it measures a different velocity, (slower).So, the questions are, which collision is inelastic/elastic and why? And what is the importance/use of the second velocity measure in part C?

These are my assumptions, Please tell me if I am correct, and why or why not am I wrong.

A ) The collision is elastic because the carts didnt stick together, and their final velocities were different.

B) The collision is elastic because the carts didnt stick together, and their final velocities were different.

C.1) The collision is inelastic because the carts stuck together, and their final velocities were the same.

C.2) The velocity measure after Cart A passed the photogate is different than when the collision took place because the carts are going slower? I don't know what the importance is though. Please, any input is appreciated! I'm deperate here, thank you!
 
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  • #2
Any help is appreciated! please :)
 
  • #3
Look up the definition of (in)elastic collisions. Also, you should make clear if they're asking about it being a theoretical elastic collision or a practical one. No practical collision is ever elastic.

Also, this belongs in the homework forum.
 
  • #4
I don't know, the prof. just said to specify if its elastic or inelastic. It's a lab experiment,

Inelastic = kinetic energy is not conserved
elastic = kinetic energy is conserved

idk how to tell if its conserved or not due to possible experimental error.
 
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  • #5
well perform some try-out calculations. If kinetic energy is conserved in a clash, does that mean they stick together or veer off independently? Then look at how your experiments are set and see which ones should theoretically make for an elastic clash and which not.
 
  • #6
only in the third collision did they stick together, only because they had magnets, but i guess that's supposed to look like an inelastic collision, but its where the kinetic energy was least conserved.also, does anyone know what the second velocity in part C means?
 
  • #7
only in the third collision did they stick together, only because they had magnets, but i guess that's supposed to look like an inelastic collision, but its where the kinetic energy was least conserved.
Why "but"? It's exactly as you should expect from an inelastic collision, think about it

Concerning the velocity in part C, again, think theoretically. What would you expect in an ideal (in)elastic situation in the second part of C. Compare that with what you got as a result.

Good luck
 
  • #8
Okay, so in the part C, after the photogate measures the final velocity, 23 m/s, ( which is the same for both carts, since they are stuck together after all), it runs across a certain short distance, until the first cart, A, runs thru the photogate (While still stuck to the second car) and the velocity is measured at 18 m/s. So, it means it slowed down?

If so, why would this second velocity be useful, the prof, said to write about its importance.

Thanks for ur help!
 
  • #9
link the velocity with the definitions of elastic and inelastic. that's all I can say without literally saying it :p remember the carts are of equal mass
 
  • #10
So, I guess it would be useful (the second velocity in the third collision), because it would help determine the velocity as its slowing down. The 18 m/s measure in Car A, would also be the same for B, then? Am I close?
 
  • #11
no.

it's not hard, think more about it, lay connections.

Read the definitions of elastic and inelastic again... Think about speed... mass... :p
 
  • #12
Is it to verify that no kinetic energy has been lost?
 
  • #13
or in this case...
 
  • #14
not conserved? gahh. I am stumped!
 
  • #15
btw these are my calculations

I.
Ps = MaVa = (0.519)(0.42) = 0.22 N*s
Ps’ = MaV’a + MbV’b = (0.519)(0) + (0.511)(0.33) = 0.17 N*s

% diff. = [|0.22 - 0.17| / (0.22 + 0.17 / (2))] x 100 = 26 %

K = (.5)(MaV^2a) = (0.5)(0.519)(0.42)^2 = 0.046 J
K’ = (.5)(MaV’^2a) + (0.5)(MbV’^2b) = (0.5)(0.519)(0) + (0.5)(0.511)(0.33)^2 = 0.028 J

% diff. = [|0.046 - 0.028| / (0.046 + 0.028 / (2))] x 100 = 49 %

II.
Ps = MaVa = (1.013)(0.46) = 0.47 N*s
Ps’ = MaV’a + MbV’b = (1.013)(0.16) + (0.511)(0.53) = 0.43

% diff. = [|0.47 - 0.43| / (0.47 + 0.43 / (2))] x 100 = 8.9 %

K = (.5)(MaV^2a) = (0.5) (1.013)(0.46)^2 =0.11 J
K’ = (.5)(MaV’^2a) + (0.5)(MbV’^2b) = (0.5)(1.013)(0.16)^2 + (0.5)(0.511)(0.53)^2 = 0.085 J

% diff. = [|0.11 - 0.085| / (0.11 + 0.085 / (2))] x 100 = 26 %

III.
Ps = MaVa = (0.519)(0.42) = 0.22 N*s
Ps’ = MaV’a + MbV’b = (0.519)(0.23) + (0.511)(0.23) = 0.24 N*s

% diff. = [|0.22 - 0.24| / (0.22 + 0.24 / (2))] x 100 = 8.7 %

K = (.5)(MaV^2a) = (0.5)(0.519)(0.42)^2 = 0.046 J
K’ = (.5)(MaV’^2a) + (0.5)(MbV’^2b) = (0.5)(0.519)(0.23)^2 + (0.5)(0.511)(0.23)^2 = 0.027 J

% diff. = [|0.046 - 0.027| / (0.046 + 0.027 / (2))] x 100 = 57%

V”a = 0.18 m/s
 
  • #16
I'm confused. Right after the collision it measures the speed and then another time a bit later?
 
  • #17
Yes, when CartB is struck by A, the photogate measures its speed (which is the same for both as they are stuck together), then Cart A passes under the photogate still stuck to B, and it measures a lower velocity. I don't know what this means.
 
  • #18
It seems that the only thing you can say, is that there is friction... huh weird :s sorry
 

1. What is the difference between elastic and inelastic collisions?

Elastic collisions are collisions in which both kinetic energy and momentum are conserved, meaning that the total energy and momentum of the system before and after the collision are equal. Inelastic collisions, on the other hand, are collisions in which kinetic energy is not conserved, meaning that the total energy of the system after the collision is less than the total energy before the collision.

2. How can I calculate the velocities of two objects after an elastic collision?

The velocities of two objects after an elastic collision can be calculated using the conservation of momentum and conservation of kinetic energy equations. The equation for conservation of momentum is m1v1i + m2v2i = m1v1f + m2v2f, and the equation for conservation of kinetic energy is ½m1v1i2 + ½m2v2i2 = ½m1v1f2 + ½m2v2f2. Solving these equations simultaneously will give you the final velocities of the two objects after the collision.

3. What is an example of a real-life elastic collision?

An example of a real-life elastic collision is a game of billiards. When the cue ball collides with another ball, momentum and kinetic energy are conserved, and the balls bounce off each other without losing any energy.

4. How can I determine if a collision is elastic or inelastic?

A collision can be determined as elastic or inelastic by calculating the total kinetic energy of the system before and after the collision. If the total kinetic energy is the same, the collision is elastic. If the total kinetic energy is less after the collision, the collision is inelastic.

5. What happens to the kinetic energy in an inelastic collision?

In an inelastic collision, some of the kinetic energy is converted into other forms of energy, such as heat or sound. This means that the total kinetic energy after the collision is less than the total kinetic energy before the collision.

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